The present invention relates generally to telecommunications and, more particularly, to improvements in communication of textual data in digital format via television, such as teletext systems.
Teletext refers generally to a system for communicating textual information in a digital format via an appropriately modified broadcast television transmitter and receiver. It is a product of the constant effort in modern civilization to provide improved communication, and specifically to find increased uses for television. Television can be used to broadcast textual information in the typical video picture format, but teletext systems increase efficiency since they communicate textual information simultaneously with typical program transmission. In the various systems throughout the world for teletext, including ORACLE in the United Kingdom, Antiope in France, and the trial teletext systems in the United States, information is transmitted from a transmission station to multiple recipients during the so-called "vertical interval." For those who are unfamiliar with television operation and this vertical interval, a television receiver includes a cathode ray tube which generates an electron beam. The electron beam scans, i.e. moves from left to right, the cathode ray tube at a predetermined rate. When the electron beam reaches the right side of the tube, it is blanked (made invisible) and caused to retrace rapidly to the left side of the tube but one line lower to continue forming an image on the television screen. When the electron beam reaches the bottom of the screen, internal blanking circuits darken the beam so that it does not produce an image, and a vertical sweep circuit causes the dark beam to retrace to the top of the screen. The time during which that darkening and retracing occurs is the vertical interval.
In the United States, the time which this vertical interval occupies is approximately 1333 microseconds, which approximates the time required for scaning twenty-one horizontal (display) lines. Thus, Twenty-one lines which are not used for viewing the television image are available for other types of communication.
Normally the teletext system communicates the textual information in a digital rather than video format, but the teletext signals are, in fact, analog representations of digital bits. The digital format is used because under present technology a piece of information such as an alphabetical letter can be telecommunicated in less time using digital techniques than video ones. Typically, at the transmitter, the textual information is digitally encoded according to American National Standards Code for Information Interchange (ASCII) format and is often protected by a Hamming code. The digital format data is transmitted during the vertical interval, and the received signal is demodulated, decoded and generally stored for subsequent use or, less often, displayed immediately on the television screen.
For normal video broadcasts, particularly in urban regions, the transmitted signal may reach a particular receiver by more than one path due to reflections of the transmitted signal off of buildings or other objects. This phenomenon, known as multipath, creates a disturbing and unpleasant video display. The distortion caused to digital data, however, is of a different and vastly more destructive character due to overlapping of digital bits of information. This is called intersymbol interference.
Linear distortion can be overcome, theoretically, as set forth more particularly by H. A. Wheeler, "The Interpretation of Amplitude and Phase Distortion in Terms of Paired Echoes," Proc. I.R.E., Vol. 27, June 1939, pp. 359-385. Paired-echo theory indicates that linear types of distortion can be approximated in terms of appropriate sets of paired echoes displaced in time about the undistorted transmitted signal. Specifically, linear distortions consist of the undistorted transmitted signal plus at least one pre-echo similar in shape to the undistorted signal, but preceeding it in time by some amount t.sub.1, and a corresponding post-echo also similar in shape to the undistorted signal but following it in time by the same amount of time t.sub.1. Generally, all linear distortions, regardless of origin, can be expressed as a sum of pre-echo and post-echoes of the undistorted signals. The process of removing distortions from a communication channel, including ghosts, is referred to as channel equalization and is implemented by devices known as channel equalizers.
Channel equalization has been proposed for teletext, but a workable system has not been developed. In S. K. Goyal and S. C. Armfield, "Reception of Teletext under Multipath Conditions,"IEEE Transactions on Consumer Electronics, Vol. CE-25, No. 3, (July 1979), Goyal and Armfield suggested the use of a transversal filter in combination with an appropriate training waveform to maintain data reliability despite multipath distortion. They simulated a deghosting system using transversal filters with variable tap weights with one of four different training wave forms. After appropriate adjustment of the tap weights, the ghosted data was passed through the filter for deghosting and consequent improvement in the eye pattern, indicating increased reliability of digital data. Goyal and Armfield conducted their experiment on an IBM 370/168 computer in conjunction with a digital frame store system with eight bit resolution and a sampling rate four times the subcarrier frequency. Following the teachings in W. Ciciora, G. Sprignoli and W. Thomas, "A Tutorial on Ghost Cancelling in Television Systems," IEEE Trans. on Consumer Elec., Vol. CE-25, No. 1, pp. 9-44 (February 1979), as well as those of H. Thedick, "Adaptive Multipath Equalization for TV Broadcasting," IEEE Trans. on Consumer Elec., Vol. CE-23, No. 2, pp. 175-181 (May 1977); E. Arnon, "An Adaptive Equalizer for Television Channels," IEEE Trans. on Comm. Tech., Vol. Com. 17, pp. 726-734 (December 1969); and K. Yamamoto, N. Yamaguchi, N. Miyata, "Ghost Reduction Systems for Television Receivers," IEEE Trans. on Consumer Electronics, Vol. CE-23, pp. 327-344 (August 1977), Goyal and Armfield found a feed-forward transversal filter operating on a baseband signal to be an acceptable deghosting method. They simulated an adaptive deghosting system using a 128-tap transversal filter with various types of training signals. The tap weights were adjusted by running the training wave form for five hundred passes to calculate the tap coeffecients. After such adjustment, they found that the adaptive equalization improved data reliability. Clearly, equipping every teletext recipient with an IBM 370 computer at a cost of over one million dollars per computer is not a feasible solution for enhancing teletext data reliability.
Another proposal or plan for equalization of digital data involves PBS' Captioning for the Deaf system, which displays textual information at the bottom of the television screen along with the transmitted video image. The PBS signal format includes an adaptive equalizer reference signal which is transmitted every eigth frame. An associated multipath filter may be microcomputer based as set forth in Thedick, op. cit. However, it would suffer significant speed limitations, as it once samples the reference pulse each time it appears--once each eight frames. Data about the reference pulse would very slowly be built up on a memory, and a filter would adaptively equalize the communication. Such a system would not effectively equalize the channel in a high data rate system.
Therefore, the main object of the present invention is to improve the reliability of textual data communications via television at a modest cost.
A further object is to provide an improved system which does not require a large and powerful computer.
A further object is to provide an improved teletext decoder arrangement.
Another object is to equalize teletext signals to overcome multipath distortion.